Goodyear Blowout

Yesterday, coming across IH 10 to San Antonio we experienced two blowouts on Goodyear ST185/80R13. Both were about an hour apart and both on the street side. The first we heard a slight pop and knew what had happened. The second we had no idea until someone pulled up along side us. This is a 3326 with double axle. I had one spare, the other we had to leave the trailer and head to a Walmart in Boerne. They only had an ST 175 and it was a Carlisle. That got us to San Antonio. Now, the big question. What could have caused two blowouts, the tires appear to be in good condition. I have one of the blownout tires which I will take to Western Auto to examine and then take the trailer when we leave on Tuesday to have all tires examined and determine what to do next. This is long and windy but are we using the right tires? Any recommendations before we head out again?

P.S. The trip has been great from McAllen to Fort Davis, will post some great photos later this week.